The present invention generally relates to radiation detection, and, more specifically, to the detection of alpha radiation at a relatively long distance from its point of emission using single and double grid detectors.
In any area where radioactive materials are handled, it is imperative, both for the protection of personnel and to avoid contamination of the environment, to continuously monitor personnel, equipment, and clothing to prevent the release of radioactive contamination. Alpha contaminants, such as plutonium, are particularly difficult to detect because of the limited penetration of alpha particles in air. Alpha particles from typical contaminants travel no more than one inch in air. It is because of this characteristic that prior alpha detectors have been useful only when used in close proximity to the point of possible emission. Currently, alpha contamination cannot be detected if it originates in a space that is too small for insertion of a conventional monitor. Prior detectors have normally been employed in personnel screening when moved slowly in close proximity to a person's body. Workers in nuclear processing facilities must place their hands and feet on sensors when moving from room to room. All of this can slow operations, as it is not currently possible to adequately screen personnel within a reasonable period of time.
It is also extremely difficult to monitor equipment for alpha contamination, again due to the limited range of alpha particles in air. Because of the monitoring difficulty, equipment that has been used in a potentially contaminated area is often classified as potentially contaminated and its further use is restricted to other controlled areas. If such equipment could be effectively monitored for contamination, the equipment could be released for use in uncontrolled areas. Previously, contamination inside assemblies has been impossible to detect without dismantling the assembly. The present invention allow contamination detection in any area that air can penetrate.
As used herein, the terms "long range," or "long distance," when referring to the detection capabilities of the present invention, shall mean detection from a range or distance of more than one (1) inch from the source of alpha radiation.
In the past, several instrument designs have been utilized to detect alpha radiation. Among these are GM tubes, ionization chambers, count rate detectors, and scintillation or gas flow proportional probes. While these instruments are capable of detecting alpha particles, they do so by directly detecting incident radiation, and must be within an inch of the source of the radiation. Also, these conventional alpha particle detectors can only scan an area approximately equal to the size of the detector.
The primary reason for an alpha particle's short flight path in air is its collision with air molecules. In almost all of these collisions, air ions are created which will have a longer life and area of influence than the alpha particles that created them. It is these ions that are detected by the present invention. The fact that the air ions have a longer range than the alpha particles relieves the necessity for having a detector moved over a person or equipment in order to detect the presence of alpha radiation.
In a copending application, Ser. No. 709,566, filed Jun. 3, 1991, a long range alpha detector is disclosed in which at least two grids are utilized. The present application discloses both single and double grid embodiments. In, for example, a two grid detector, one grid is the signal grid and the other is the voltage grid. As the signal grid is at virtual ground, no (or at most a very small) electric field exists between the signal grid and the grounded enclosure, and ions are not disturbed in this area. However, a significant electric field exists between the voltage grid and the signal grid. This field sweeps ions of one charge, depending on the polarity of the voltage grid, onto the signal grid, where they are detected by a current meter. Ions of the opposite polarity are swept onto the voltage grid, and their signal is lost.
With a single grid detector, according to the present invention, voltage is applied to the grid, creating an electrical field between the grid and the grounded enclosure. This sweeps ions of one polarity to the grid, while ions of the other polarity are lost to the enclosure. Using only one grid simplifies construction and reduces leakage currents because of the reduced number of grid support insulators. In a related embodiment, a combination detector combines two grids in a circuit which allow signal contribution from both polarities of ions.
It is therefore an object of the present invention to provide apparatus for the long range detection of alpha radiation using only a single grid.
It is another object of the present invention to provide apparatus for long range alpha radiation detection using two grids of opposite polarity allowing detection of alpha created ions of both polarities.
It is a further object of the present invention to provide apparatus for detection of alpha particle contamination within pipes and ducts.
It is a still further object of the present invention to provide apparatus for monitoring alpha radiation inside large tanks.
It is another object of the present invention to provide apparatus for long range monitoring of alpha radiation from soil core samples and other standard sized objects.
Additional objects, advantages and novel features of the invention will be set forth in part in the description which follows, and in part will become apparent to those skilled in the art upon examination of the following or may be learned by practice of the invention. The objects and advantages of the invention may be realized and attained by means of the instrumentalities and combinations particularly pointed out in the appended claims.